Temperature-sensitive mutants of Simian Virus 40 (SV40) are being isolated and characterized. These mutants are used for the dissection at the molecular level of the SV40-host cell interactions that occur during productive infection in monkey cells, semiproductive infection in human cells, and abortive infection in mouse cells. Special emphasis is placed on the regulatory mechanisms of normal and SV40-transformed mouse, monkey, and human cells. The regulation of the SV40 infectious cycle and the SV40-transformed cell by the cellular cyclic AMP system is being investigated with the mutants and dibutyryl cycle adenosine 3':5'- monophosphate.